According to Mac Rumors, this exact date of release is based off of the fact that Apple retail stores will be conducting store updates the night of July 24. It speculates that these updates will include installations of OS X 10.8 Mountain Lion on all store Macs, as well as the addition of promotional information throughout the store.

OS X 10.8 Mountain Lion will be Apple's ninth operating system update in the OS X series (unless you count the public beta version "Kodiak," which would make this the 10th release).

This is actually the downside we're seeing to someone who creates both the hardware and software. They feel that can make up lost sales on the software front due to some customers buying new hardware to get future OS updates. Were as Microsoft tries to make their OS as compatible as possible with older systems because this increases potential sales.